Presence of cytosolic retinoic acid binding proteins (cRABP) sedimenting as a 2S component in carcinogen-induced rat mammary adenocarcinomas has been reported previously. Following incubation of isolated nuclei with (3H) retinoic acid bound cRABP 2S binding component was detected in the 0.4 M KCl extract of the nuclei. Incubation of nuclei with (3H) retinoic acid did not result in any significant binding. Temperature activation of (3H)RA-cRABP by incubating the complex at either 25 or 30 degrees C for 30 min. did not alter the sedimentation coefficient; moreover, it did not enhance the nuclear binding. Influence of retinoids on differentiation of mouse mammary glands in organ cultures was also investigated. All-trans-retinoic acid at 1 by 10 to the minus 7th power M concentration inhibited the ductal branching and hormone-induced proliferation. Inhibition of DNA synthesis by all-trans-retinoic acid and N-(4-hydroxyphenyl) retinamide was observed; however, retinyl actate was ineffective in inhibiting DNA synthesis.